1. Field of the Invention
The present invention relates to an ultrasonic probe used for an ultrasonic test apparatus and, more particularly, to an ultrasonic probe system which is constituted by a stacked piezoelectric element and is capable of transmitting/receiving ultrasonic waves having different frequencies.
2. Description of the Related Art
A detailed description of the prior art is available from the following references:
(1) Japanese Patent Disclosure (Koukai) No. 60-41399
(2) Japanese Patent Disclosure (Koukai) No. 61-69298
An ultrasonic probe has a probe head mainly constituted by a piezoelectric element. This ultrasonic probe is used to obtain image data representing the internal state of a target object by radiating ultrasonic waves onto the target object and immediately receiving waves reflected from interfaces of the target object which have different acoustic impedances. An ultrasonic test apparatus using such an ultrasonic probe is used in practice as, e.g., a medical diagnosing apparatus for examining the inside of a human body, or an industrial test apparatus for inspecting flaws in welded metal portions.
The diagnosing function of a medical diagnosing apparatus has been greatly improved owing to the development of "the color flow mapping (CFM) method" in addition to photography of a tomographic image (B mode image) of a human body. In this CFM method, blood flow rates in a heart, a liver, a carotid artery, and the like as targets are two-dimensionally displayed in color by using the Doppler effect. Recently, the CFM method has been used to diagnose all kinds of internal organs of a human body, such as the uterus, the kidney, and the pancreas. Further studies of the CFM method are now in progress to allow observation of even the movement of a coronary blood flow.
With regard to the above-mentioned B mode image, i.e., a tomographic image of a human body, it is required that a high-resolution image be obtained with high sensitivity to allow an operator to clearly observe a physical change or a cavity as a slight morbid alteration. In the Doppler mode for acquiring a CFM image or the like, since echoes (waves) reflected by, e.g., microscopic blood cells, each having a diameter of several .mu.m, are used, the resulting signal level is lower than that obtained in the B mode described above. For this reason, high-sensitivity performance is especially required. In many cases, a reference frequency in this Doppler mode is set to be lower than the center frequency in the frequency band of an ultrasonic probe. This is because a frequency component exhibiting small attenuation is used to suppress the influences, of ultrasonic attenuation through a living body, which cause a decrease in S/N ratio. Therefore, providing that ultrasonic waves having two different types of frequency components can be transmitted/received by a single ultrasonic probe, both a high-resolution B mode image constituted by high-frequency components and a high-sensitivity Doppler image constituted by low-frequency components can be obtained. As probes having such functions, "duplex type ultrasonic probes" are available from various manufacturers. A duplex type ultrasonic probe is designed such that two types of vibrators having different resonance frequencies are arranged in one ultrasonic probe. Since an ultrasonic probe of this type uses different types of vibrators, ultrasonic transmission/reception planes are set at different positions. For this reason, tomographic images of the same portion cannot be observed. Under the circumstances, a method of transmitting/receiving ultrasonic waves in two types of frequency bands by using a single vibrator has been proposed, which uses a stacked piezoelectric element disclosed in Japanese Patent Disclosure (Koukai) No. 60-41399. Two types of frequency bands can be separated from each other by using a combination of an ultrasonic probe of this type, a driving pulser, and a filter. As a result, a B mode signal and a Doppler signal can be respectively acquired from high-frequency components and low-frequency components. However, in the ultrasonic probe having the above-described arrangement, since the electromechanical coupling efficiency of one piezoelectric element is divided into substantially halves, the high-frequency side frequency band is narrowed, and the remaining time (duration) of an echo signal is prolonged. For this reason, even if a B mode image is obtained by using high-frequency components to ensure high resolution, the resulting resolution is not so high as expected. That is, there is a room for improvement in this point. In addition, since low-frequency components are generally decreased in number as the frequency band becomes narrower, the S/N ratio is decreased, resulting in insufficient penetration. This is because an echo signal reflected by a portion located deep in a living body is mainly constituted by frequency components lower than the center frequency of transmitted ultrasonic waves. The specific band width of frequency components, which is required to obtain a good B mode image, is 40% or more of its center frequency. Assume that a single-layered piezoelectric element is used. In this case, a specific band width with respect to a center frequency at -6 dB is 40 to 50% in one-layer matching, and 60 to 70% in two-layer matching. In contrast to this, if the stacked piezoelectric element having the above-described arrangement is used, specific band widths of 25% and 35% are respectively set in one-layer matching and two-layer matching. That is, if only the stacked piezoelectric element is used, the obtained specific band width is only about 1/2 that obtained when the single-layered piezoelectric element is used.
An increase in sensitivity may be realized by increasing a driving voltage. This method, however, is also limited by the problem of heat generated by a piezoelectric element. Another problem posed in the method of obtaining two types of frequency bands by using a single ultrasonic probe is that the same portion cannot be observed because of the use of a plurality of vibrators having different resonance frequencies. As described above, in order to solve this problem, the stacked piezoelectric element is disclosed in Japanese Patent Disclosure (Koukai) No. 60-41399, which is obtained by stacking piezoelectric elements, each having substantially the same thickness as that of the single-layered piezoelectric element and consisting of substantially the same material as therefor. This element, however, poses the problem of a narrow specific band of high-frequency components.
As described above, when ultrasonic waves in two types of frequency bands are to be acquired by one ultrasonic probe, the same portion of a target object cannot be observed with a probe head constituted by a plurality of vibrators having different resonance frequencies. In the stacked piezoelectric element disclosed in Japanese Patent Disclosure (Koukai) No. 60-41399 to solve this problem, which is obtained by stacking layers, each having substantially the same thickness as that of the single-layered piezoelectric element and consisting of substantially the same material as therefor, the specific band of high-frequency components is too narrow.